The goal with this drill is to teach the wrestlers how to float on top with a half nelson while the bottom guy tries to stand up, does a forward roll, granby, side-roll, whatever they can to get your hips out of position. The goal is for the top guy to stay on top of the bottom guy, knees bent, on either side of the hips, pinching, and just roll with the entire time.
The biggeset issue tends to be the top guy will fall off because their hips are too low. So, get your hips up higher than you think you need to. With a half and a tight waist, you shouldn't get so high that you fall off in front, so this is relatively safe.
30 second goes, if the top guy falls off, they reset. Bottom guy shouldn't be getting to their feet, if that happens too much then tell them they can't do that.